The preparation of surface relief structures in photosensitive materials is well known. Over the years, the art has learned how to reduce the dimensions of the relief lines or holes (viewed normal to the surface) to the point where they are measured in terms of micrometers and fractions of micrometers.
Extensive studies have been made, in particular, of periodic one dimensional structures prepared in positive type photosensitive materials or photoresists. (It will be appreciated that the terms "one dimensional" and "two dimensional" are terms of the art describing the relief structure from a view normal to the surface). One type of photoresist contains a photosensitive polymer which then exposed to light becomes soluble in an appropriate water base developer. After exposure and development, the initially flat surface of the photoresist becomes a surface relief structure whose depth varies depending upon the photoresist that has been etched away by the developer in proportion to the exposure light intensity. When exposed to an intensity variation that is periodic, such as a light interference pattern, a periodic surface profile will be formed which is everywhere proportional to the initial interference intensity pattern. Precise relief structures of this type can be made easily over relatively large areas using laser interference techniques.
It is known in the art to form relief diffraction gratings employing laser interference techniques. In general, such gratings are formed by exposing a photosensitive material such as a photoresist to two coherent interfering laser beams (recording beams) whose wavefronts are substantially plane and parallel. When such beams interfere, there is produced a stationary periodic fringe pattern consisting of maxima and minima of beam intensity. The spacing between adjacent maxima (or minima) is determined by the angle between the beams and by the wavelength of the exposing light. Depending upon the optical system used, substantially any spacing can be obtained down to about half the wavelength of the exposing light. The photosensitive material will thus be exposed to a periodic variation in intensity across its surface.
The above description applies to the formation of straight line gratings (one-dimensional gratings); that is, the maxima or minima of the developed image appear as straight parallel lines when viewed normal to the surface. A crossed grating (two-dimensional grating) can be obtained by rotating the photosensitive material 90.degree. about an axis perpendicular to the center of the surface subsequent to the first exposure and exposing a second time. In this case, the surface is subjected to two periodic intensity variations at right angles to each other. Upon development, the resulting relief structure will consist of a rectangular array of peaks and valleys; in the case of a positive photoresist, the peaks correspond to the areas where the combined intensity of the two exposures was the least, or where there was no exposure, and the valleys to the areas where the exposure was the greatest.
Variations in the symmetry of the above described array are also possible. For example, by changing the angle between the two beams after the first exposure, one obtains a different grating spacing for each of the two perpendicular orientations. This can lead to points of intersection which are oblong instead of round. Alternatively, if the exposure plate is rotated to form an angle other than 90.degree. between the two exposure positions, one would obtain a diamond-shaped rather than a square point of intersection array.
Copending application of James J. Cowan, Arthur M. Gerber and Warren D. Slafer, Ser. No. 234,959, now U.S. Pat. No. 4,402,571 filed Feb. 17, 1981 (common assignee), is directed to a method of producing accurate relief patterns in a photoresist in which individual features of the relief patterns can be of submicron size and in which the size and shape of the relief patterns are controlled, to a great extent, by manipulation of the exposure and development parameters of the photoresist as distinguished from the pattern or image to be projected onto the photoresist. The method involves exposing the photosensitive material at a first position to a laser interference pattern, rotating said material around an axis perpendicular to its surface to a second position and exposing said material at said second position for a laser interference pattern, wherein at least one and preferably both of said exposures is individually below the effective threshold for linear response of said material, the points of intersection of the two fringe patterns being exposed above said threshold as a result of the combined exposures, and developing said material.
A disadvantage to making patterns employing crossed grating methods arises from the sequential nature of the exposures. At the cross-over points of the maxima of each pattern, the exposure is maximum, and the subsequent etching is deepest; at the minima, the exposure is zero and there is substantially no etching at these points. But at the intermediate points the exposure is half of the maximum and subsequent etching of the exposed photosensitive material may lead to the formation of saddle points. Thus instead of obtaining an array of very deep holes in an otherwise flat surface, often an array of not very deep holes on a highly undulating surface is formed, which may be an undesirable condition for many applications. Furthermore, the array of holes is not close packed; in most cases the holes occupy only about half of the total surface area.